Recent water quality trends and a comparison to sediment-core records for two riverine lakes of the Upper Mississippi River basin: Lake St. Croix and Lake Pepin |
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| Authors: | Brenda Moraska Lafrancois Suzanne Magdalene D Kent Johnson |
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| Institution: | (1) National Park Service, U.S. Department of Interior, 16910 152nd St. N., Marine on St. Croix, MN 55047, USA;(2) St. Croix Watershed Research Station, Science Museum of Minnesota, 16910 152nd St. N., Marine on St. Croix, MN 55047, USA;(3) Metropolitan Council Environmental Services, 2400 Childs Road, St. Paul, MN 55106, USA |
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| Abstract: | Long-term water quality monitoring data from two riverine lakes in the Upper Mississippi River basin, Lakes St. Croix and
Pepin, were analyzed to compare the long-term average water quality conditions and land use distributions, water quality trends
and loads at lake inlets and outlets, trends from long-term versus short-term monitoring records, and the ability of paleolimnological
cores to accurately infer lake water quality conditions. During the 1976–2004 period, the long-term average concentrations
of nutrients, suspended solids, and chlorophyll-a were consistently lower at the Lake St. Croix inlet versus the Lake Pepin inlet, which drains a greater proportion of urban
and agricultural runoff. Despite these differences, nutrient trends were similar at the inlets to both lakes; reductions in
total phosphorus and ammonium concentrations were attributed to improvements in point source technologies, whereas increasing
nitrate concentrations were attributed to both point source changes and nonpoint source increases. Despite improvements in
several water quality variables, nitrate concentrations are increasing in both lakes, sediment trends indicate persistent
nonpoint source inputs to Lake Pepin, and current total phosphorus concentrations remain well above pre-1950s levels in both
lakes. Since urban development and agriculture are increasing in the Lake St. Croix and Lake Pepin Watersheds, continued point
source regulation and additional nonpoint source control efforts will be needed to further improve water quality in these
lakes. The 1976–2004 trends for most water quality variables were similar at inlet versus outlet sites on Lake St. Croix.
Trends at Lake Pepin inlet versus outlet sites were less similar, but data availability limited the comparison to the 1993–2003
period. While the truncated data record highlighted short-term trends in both lakes, the full data record was most useful
for exploring general patterns in water quality. Length of monitoring record affected our ability to detect trends at the
inlets to both lakes, and altered the magnitude of detected trends. During the two decades of the 1980s and 1990s, paleolimnological
estimates of retained phosphorus loads were similar to those estimated from recent water quality monitoring. These similarities
support the use of paleolimnological approaches to infer past water quality conditions in Lakes St. Croix and Pepin.
This is one of eight papers dedicated to the “Recent Environmental History of the Upper Mississippi River” published in this
special issue of the Journal of Paleolimnology. D. R. Engstrom served as guest editor of the special issue. |
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| Keywords: | Water quality monitoring Trend analysis Lake coring Nutrient loading Wastewater Land-use |
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